10 Search Results

Atomically precise cerium oxo clusters offer a platform to investigate structure–property relationships that are much more complex in the ill-defined bulk material cerium dioxide. We investigated the activity of the MCe₇₀ torus family (M = Cd, Ce, Co, Cu, Fe, Ni, and Zn), a family of discrete oxysulfate-based Ce₇₀ rings linked by monomeric cation units, for CO oxidation. CuCe₇₀ emerged as the best performing MCe₇₀ catalyst among those tested, prompting our exploration of the role of the interfacial unit on catalytic activity. Temperature-programmed reduction (TPR) studies of the catalysts indicated a lower temperature reduction in CuCe₇₀ as compared to CeCe₇₀.more » In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that CuCe₇₀ exhibited a faster formation of Ce³⁺ and contained CO bridging sites absent in CeCe₇₀. Isothermal CO adsorption measurements demonstrated a greater uptake of CO by CuCe₇₀ as compared to CeCe₇₀. The calculated energies for the formation of a single oxygen defect in the structure significantly decreased with the presence of Cu at the linkage site as opposed to Ce. Furthermore, this study revealed that atomic-level changes in the interfacial unit can change the reducibility, CO binding/uptake, and oxygen vacancy defect formation energetics in the MCe₇₀ family to thus tune their catalytic activity.« less

Oxidative dehydrogenation of propane (ODHP) to propylene could have a significant impact on the production of this critical chemical intermediate, if appropriate catalysts can be discovered. Recently, heterogeneous catalysts based on boron (oxides and nitrides) have been demonstrated to be promising for ODHP, but their active sites have not been conclusively identified. Here, we report that the deposition of differently sized boronic acids into the micropores of silica supports results in different distributions of surface borate species after calcination. Furthermore, these materials, in turn, display a wide range of rates in ODHP but similar selectivity, suggesting that they differ onlymore » in the numbers of active sites. Features identified by in situ Raman, IR, and magic-angle-spinning ¹¹B solid-state NMR spectroscopies are compared to catalyst activity. This correlation identifies the S2 borate species, a hydroxylated nonring boron, as the likely active site and provides a target for directed syntheses of future catalysts.« less

Selective oxidation of cyclohexene to 2-cyclohexen-1-one over titania supported vanadia (VO_x/TiO₂) has been studied using temperature dependent in-situ FTIR spectroscopy in both the presence and absence of oxygen. The VO_x/TiO₂ samples were prepared using one atomic layer deposition (ALD) cycle and characterized by Raman spectroscopy. In-situ FTIR data for the oxidation of cyclohexene and perdeuterocyclohexene allow for the formulation of a molecular level reaction mechanism, which is initiated by the transfer of an allyl hydrogen. Oxidation of perdeuterocyclohexene provides a direct probe of the formation of OD and HDO moieties that support the involvement of specific steps in the proposedmore » mechanism. The presence of gas phase oxygen does not lead to a change in the products versus anaerobic conditions. However, gas phase oxygen is significantly incorporated in the CO₂ over-oxidation product above ~250 °C. Data were also obtained with cyclohexene epoxide as the reactant in an effort to determine whether there is a parallel reaction pathway, which is initiated by C=C activation in cyclohexene, that involves cyclohexene epoxide as an intermediate. Furthermore, though a minor pathway involving a cyclohexene epoxide intermediate cannot be ruled out, these data demonstrate that, under experimental conditions, the dominant pathway from cyclohexene to cyclohexene-1-one is initiated by an allyl-H activation step and does not involve an epoxide intermediate.« less

Carbon dioxide hydrogenation to CO via the reverse water gas shift (RWGS) reaction is one route to integrate CO₂ utilization into the chemical industry. TiO₂ supported Cu catalysts are known to be active for RWGS, but Cu is shown here to behave differently on TiO₂ nanotubes (TiNT) vs TiO₂ nanoparticles (TiNP). Whereas nanoparticle supports give low rates that are hardly changed by added Cu, the nanotube supports yield much higher activity and three distinct behaviors as the Cu surface density increases. At low surface densities (0.3 Cu/nm²), active Cu-O-Ti sites are created that have low apparent activation energies. At highmore » surface densities (6 Cu/nm²), Cu nanoparticles on TiNT are formed, and reaction barriers are lowered when both Cu and TiNT surfaces are accessible. At intermediate surface densities, metallic Cu domains are engulfed by a TiOx overlayer formed during H₂ pretreatment, akin to those formed by classical strong metal support interactions (SMSI). These reduced layers are markedly more active for RWGS than the initial TiNT surfaces, but have similar activation barriers, which are higher than those for which both Cu and TiNP surfaces are exposed. These catalytic findings are supported by computational modeling, in situ IR, UV–vis, and X-ray absorption spectroscopies, and they provide insight into an important reaction for CO₂ utilization.« less

Gas phase acceptorless dehydrogenative coupling (ADC) of ethanol to ethyl acetate is investigated over bulk MoS₂. Yields of up to 56% are achieved at gas hourly space velocity of 1700 h^-1 and 260 °C using a high surface area co-precipitated MoS2 material. Addition of water to the ethanol feed decreases overall conversion but increases selectivity to acetic acid. After a short activation period, the catalyst is stable over reaction times exceeding 13 h. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as a probe molecule and X-ray absorption spectroscopy (XAS) are used to measure structural disorder within MoS₂ samples.more » Samples with larger amounts of structural disorder are demonstrated to have enhanced dehydrogenation activity per total surface area.« less

Collapsed titania nanotubes (cTiNT) were synthesized by the calcination of titania nanotubes (TiNT) at 650 °C, which leads to a collapse of their tubular morphology, a substantial reduction in surface area, and a partial transformation of anatase to the rutile phase. There are no significant changes in the position of the XPS responses for Ti and O on oxidation or reduction of the cTiNTs, but the responses are more symmetric than those observed for TiNTs, indicating fewer surface defects and no change in the oxidation state of titanium on oxidative and/or reductive pretreatment. The interaction of H₂O and CO₂ withmore » the cTiNT surface was studied. The region corresponding to OH stretching absorptions extends below 3000 cm-1, and thus is broader than is typically observed for absorptions of the OH stretches of water. The exchange of protons for deuterons on exposure to D₂O leads to a depletion of this extended absorption and the appearance of new absorptions, which are compatible with deuterium exchange. We discuss the source of this extended low frequency OH stretching region and conclude that it is likely due to the hydrogen-bonded OH stretches. Interaction of the reduced cTiNTs with CO₂ eads to a similar but smaller set of adsorbed carbonates and bicarbonates as reported for reduced TiNTs before collapse. Implications of these observations and the presence of proton sources leading to hydrogen bonding are discussed relative to potential chemical and photochemical activity of the TiNTs. These results point to the critical influence of defect structure on CO₂ photoconversion.« less

We report on a combined experimental and theoretical study of the acid catalyzed dehydration of d-fructose in dimethylsulfoxide (DMSO) using; Amberlyst 70, PO₄^3–/niobic acid, and sulfuric acid as catalysts. The reaction has been studied and intermediates characterized using; ¹³C, ¹H, and ¹⁷O NMR, and high resolution electrospray ionization mass spectrometry (HR ESI–MS). High level G4MP2 theory calculations are used to understand the thermodynamic landscape for the reaction mechanism in DMSO. We have experimentally identified two key intermediates in the dehydration of fructose to form HMF that were also identified, using theory, as local minima on the potential surface for reaction.more » A third intermediate, a species capable of undergoing keto–enol tautomerism, was also experimentally detected. However, it was not possible to experimentally distinguish between the keto and the enol forms. These data with different catalysts are consistent with common intermediates along the reaction pathway from fructose to HMF in DMSO. Lastly, the role of oxygen in producing acidic species in reactions carried out in DMSO in presence of air is also discussed.« less